The kidneys serve various functions for maintaining a healthy condition of the human body. As one aspect the kidneys control the fluid balance by separating any excess fluid from the blood volume of the patient. Second they serve to purify the blood from waste substances such as urea or creatinin. Further they also control the level of certain substances in the blood such as electrolytes to ensure a healthy and necessary concentration level.
In case of renal failure excess fluid accumulates in body tissue and causes an increasing stress to the circulation/vascular system. This excess fluid has to be withdrawn from the patient using ultrafiltration. If an insufficient amount of fluid is withdrawn, the long term consequences may be severe and may lead to an increased blood pressure and heart failure. The risk of a heart failure is increased for dialysis patients and it is assumed that excess fluid is an important factor for this. Removing an excessive amount of fluid is also dangerous as the dialysis patient will become dehydrated, resulting in a hypotension.
The dry weight (for simplicity the terms weight and mass shall be used synonymously in this application—in correspondence with medical practice) defines the weight of the patient that would be reached, if the kidneys were functioning normally. In other words the dry weight represents the optimum target weight, or the fluid status, that should be reached to minimize the cardiovascular risk. The dry weight has always been a difficult to address problem in clinical practice, as quantitative procedures for determination have not been available. Currently the dry weight is often approached using indirect indicators such as blood pressure, echocardiography, and subjective information such as X-ray imaging. In addition it has been difficult to compose a set of conditions that is generally accepted as a dry weight standard.
A promising approach to assess the fluid status of a patient involves bioimpedance measurements. A low alternating current is applied to the patient using two or more electrodes, that are to be attached to the patient, and the corresponding difference of the electrical potential is measured. The various fluid compartments contribute differently to the measured signal. The usage of multiple frequencies allows to determine the intracellular volume (ICV) and the extracellular volume (ECV). To this end a typical model to analyze the bioimpedance measurement data includes a chain of sub-models. In a first step a spectrum, e.g. between 5 kHz and 1 MHz is applied and the complex impedances
      Z    ⁡          (              j        ⁢                                  ⁢        ω            )        =            u      ⁡              (                  j          ⁢                                          ⁢          ω                )                    i      ⁡              (                  j          ⁢                                          ⁢          ω                )            are recorded for the spectrum, resulting in a semi-circle like curve in the complex impedance plane. As a next step or submodel the semicircular impedance spectrum is modelled using an equivalent circuit such as an equivalent circuit including a resistance RE modelling the extracellular current path and a resistance RI and a capacitor together modelling the intracellular current path. Also more complex equivalent circuits having more than one resistor/capacitor combination have been proposed. A suitable chain of submodels for determining the overhydration of a patient is described in “Modellbasiertes impedanzmessendes Assistenzsystem bei der Diagnose and Therapie von Mangelernährung”; VDI Verlag 2009, ISBN 978-3-18-327517-5.
Based on the determined electrical resistance RE and RI and anthropomorphic parameters like height h, weight m and body mass index BMI the extracellular volume (ECV) intracellular Volume (ICV) may be derived using the following formulas:
                    E        ⁢                                  ⁢        C        ⁢                                  ⁢        V            =                                    k            ECV                    (                                                    h                2                            ⁢                              m                                                    R              E                                )                          2          /          3                      ;          kECV      =                        0.188                      B            ⁢                                                  ⁢            M            ⁢                                                  ⁢            I                          +        0.2883                                          I          ⁢                                          ⁢          C          ⁢                                          ⁢          V                =                                            k              ICV                        (                                                            h                  2                                ⁢                                  m                                                            R                I                                      )                                2            /            3                              ;              kICV        =                              5.8758                          B              ⁢                                                          ⁢              M              ⁢                                                          ⁢              I                                +          0.4194                      ,  
Based on the determined intracellular volume (ICV) and extracellular volume (ECV) it is possible to determine the hydration state in terms of an amount of excess fluid or a dehydration. One example of such an arrangement or device is described in the international patent application WO 2006/002685. This device also allows determining the body composition in respect to other volume compartments of the patient, in particular the fraction of lean and adipose tissue. Thus it is also possible to assess the nutrition status of a patient.
The above mentioned chain of models relies on a measurement which is based on multiple frequencies, requiring a relatively complex hardware equipment both for sweeping through the frequency spectrum and for analyzing the results of the measurement. Also the handling of the spectroscopic hardware equipment and the performing of the bioimpedance measurement usually requires the presence of trained staff. Therefore the models described above are mainly available for patients in clinics.
Therefore it is an object of the present invention to overcome this problem and to provide a method and a device for determining the hydration or nutrition status suitable for ambulatory patients.